Portable systems that operate from a battery and/or from power harvested from the environment typically need to consume small amounts of energy to prolong the system lifetime for a given amount of available energy. The energy budget for a portable system is increasingly important in a widening set of applications due to a combination of requirements for smaller size (less battery volume, so less energy available), longer lifetimes (make energy last longer), and/or more functionality (do more with the same amount of energy).
Many portable electronic devices such as wireless sensor nodes further typically spend a large fraction of their time in sleep modes waiting for external or internal stimuli to awaken them. During these sleep (or standby) modes, many devices use a stable clock source for keeping time to reduce the cost of re-synchronizing to other radios, among other reasons. During active modes, an accurate timing reference is used for bias precise data sampling, RF modulation, and synchronous digital computation, among other reasons.
One known approach to provide an accurate clock source includes using a crystal oscillator (XTAL). XTAL-based oscillators can consume an appreciable portion of available system power, especially during standby modes. For example, an energy harvesting body sensor network (BSN) SoC (system on a chip) having a 200 kHz XTAL consumes 19 μW while measuring ECG, extracting heart rate, and sending RF packets every few seconds. F. Zhang, Y. Zhang, J. Silver, Y. Shakhsheer, M. Nagaraju, A. Klinefelter, J. Pandey, J. Boley, E. Carlson, A. Shrivastava, B. Otis, and B. H. Calhoun, “A Battery-less 19 μW MICS/ISM-Band Energy Harvesting Body Area Sensor Node SoC,” ISSCC Dig. Tech. Papers, pp.298-299, 2012, which is incorporated by reference. In this example, over 2 μW of the total power consumption is consumed by the 200 kHz XTAL. Id.
Thus, a need exists for a method to produce an accurate clock signal from a XTAL oscillator at much lower power levels that are compatible with miniaturized ultra low power electronics.